1. Field of the Invention
The present invention relates to means and methods for the digital combination of a plurality of speaker voice channels and, in particular, to means for obtaining the combination of a plurality of delta modulated voice channels at the original speaker data bit rate.
2. Description of the Prior Art
Intercom systems, when employed for shipboard command and control and other military-type uses, must maintain secure interfaces with speaker and listener terminals. Often such security is achieved at a relatively high cost in terms of system complexity. Present day Naval intercom systems, for example, are secure when operated in the range of audio frequencies. It has been found that the maintenance of such security has complicated attempts to take advantage of digital voice transmission using delta modulation in such systems.
Delta modulation of voice data, presently utilized in transcontinental telephone systems, involves the transformation of a voltage waveform, which is the amplified output of a microphone, into a digital data bit stream. The bit stream consists of "ones" and "zeros". Transformation occurs by detecting the rate of change of the voltage waveform with respect to time. When this rate of change exceeds a positive threshold X the bit stream will contain consecutive "ones"; likewise, when the (negative value of) the rate of change of the voltage waveform with respect to time exceeds a (negative) threshold Y the bit stream will contain consecutive "zeros". When the rate of change does not exceed either the positive or negative threshold the bit stream consists of alternating "ones" and "zeros". Such an alternating bit stream is known as an "idle" waveform.
In the prior art two principal methods, a multiplex method and an analog method, have been employed for the conferencing of delta modulated digital data. In the former the multi-station voice data is multiplexed onto a bus at N-times the individual speaker bit rate (N is the number of speaker stations serviced). The greater-than-speaker bit rate data as multiplexed is then transmitted to the listening station(s).
The analog method involves the conversion of the delta modulated data from each individual speaker station to an analog waveform. The reconstituted analog waveforms are then mixed by conventional techniques. The composite analog waveform which is generated is then converted to delta modulated data for transmission to a listening station at the original delta modulated data bit rate.
The present invention accepts delta modulated voice data from multiple sources and performs multi-station combination while the data is encoded as digital data bits. The combined data is then converted to a single delta modulated line per listener. The data remains in a digital format within the system as all multiplexing operations are performed within a central switching unit. This allows the transmission of the combined data to the listening station(s) at the original delta modulated data bit rate. No conversion to analog waveform occurs within the system. By elimination of the conversion to an analog waveform the amount of system circuitry required is greatly reduced. The maintenance of the original delta mmodulation bit rate reduces the bandwidth of the transmitted output and thus enhances the security of the system.